I'm thinking to modify this spring-piston rifle into something more powerfull.

The first thing i could think is stretching the spring. Though that was suprisingly difficult. I took it out once and barely managed to put it back. Stretching it was even more difficult. I think i got it just a little bit longer.

The next thing I have tried was to put some nitro behind the pellet. That made it go a little faster.

The other option I have in mind is to make a custom reservoir for it and convert it into a pneumatic.